The use of vibrating screens for separating mineral ores is well known. Vibrating screens are used in three related applications: classification; dewatering; and media recovery. Classification is a process of separating feed material into two or more controlled size ranges. Dewatering separates water from the feed material and typically requires a much finer screen than classification. Media recovery is similar to dewatering as the screen is used to recover fluid from the feed material.
Historically the vibrating screens use screen decks of woven wire mesh to filter material by size. The woven wire mesh defines a large number of openings of a fixed size that allow material below the fixed size to pass through while other material moves across the screen.
The sized material is referred to as underflow and the remaining material is referred to as overflow. There is typically a small amount of oversize material in the underflow due primarily to damage in the panels. There is usually a larger amount of undersize material in the overflow due to limits in the performance of the vibrating screen. The measure of performance of the vibrating screen depends on the purpose of the screen but common measures of performance are ratios between overflow, underflow, undersize and oversize. For instance, the quantity of undersize in overflow compared to the quantity of oversize in underflow may be an important consideration in some applications.
An important factor in screen performance is ‘open area’. Open area is the percentage of the screen deck that is open to allow material to pass through. The open area is related to the slot size of the screen deck which is determined by the size of material to be screened. Typically slots of width from 500 micron to 2 mm will be up to about 15 mm long; slots with width from 6 mm to 12 mm will be about 30 mm to 35 mm long; slots with width around 15 mm to 20 mm will be 45 mm to 50 mm long and for slots with width above 20 mm the length may be 100 mm to 150 mm. In other words, the same basic screen design will generally have increasing open area with increasing slot size up to a limit where wearability becomes a problem. Increased open area means decreased ligament size (the panel material between slots) which means greater chance of damage due to wear. In conventional screen designs there is a trade off between wearability and open area.
Screen panels often operate below stated performance because of reduced open area due to pegging. Pegging of screen openings occurs when material becomes stuck in the openings. Attempts have been made to address this problem by making the mesh flexible so that plugged material is released during vibration of the screen. For instance, U.S. Pat. No. 4,120,785, assigned to Mitsuboshi Belting Limited, describes a screen for a vibratory screening machine comprising a mesh of rubber members. Each rope member comprises a rubber covered tensile member of wire. This solution incurs significant manufacturing cost.
Another variation has been to replace the wire mesh with polyurethane panels that provide better wear characteristics than wire mesh with similar flexibility. The polyurethane panels are made with slots having a relief angle (wider on the underside than the upper side) to assist with release of pegging material.
A typical polyurethane panel is shown in U.S. Pat. No. 4,661,245, assigned to Fioris Pty Ltd of Australia. Each panel has a grid of square openings in a moulded polyurethane block. The panels can conversely be thought of as a grid of polyurethane ribs defining the openings. When considered in this manner it can be seen that the polyurethane panel is equivalent to the woven wire mesh but with improved wear properties. However, the polyurethane panels have typically not achieved the same open area as wire mesh.
The polyurethane screen panels are made by injection moulding or air casting. In either case a mould must be produced and the screen panel cast from the mould. Persons skilled in the field will appreciate that each mould is expensive to produce. Furthermore, the challenge of machining the mould limits the design to simple aperture shapes, such as the square apertures described in the Fioris patent.
To further improve screen panel performance it was seen as desirable to manufacture aperture shapes other than square. Apertures formed from zigzag ribs are described in U.S. Pat. No. 4,892,767, assigned to Screenex Wire Weaving Manufacturers (Proprietary) Limited of South Africa. The Screenex patent describes a screen panel moulded from polyurethane and comprising a plurality of zigzag ribs extending between sides opposing of the panels so as to define a regular arrangement of diamond-shaped screening apertures. The ribs are resiliently deformable to facilitate unblocking of the apertures during screening operations. However, it will be appreciated from a careful consideration of the patent that the apertures are still square but merely rotated 45 degrees.